Air-fuel injection system for a turbojet engine

ABSTRACT

An improved swirler system for a turbojet engine combustion chamber is disclosed in which stationary first vanes and moveable second vanes are oriented at different angles with respect to a radius of the fuel injector system. The different angles allow the angle of the incoming air to be adjusted between full power and idle conditions so as to vary the conial shape of the atomized air-fuel mixture according to the operating conditions of the engine. Radial and axial inlet duct portions serve to attenuate the wakes of the air flow generated by the vanes prior to the air passing into the combustion chamber.

BACKGROUND OF THE INVENTION

The present invention relates to an air-fuel injection system for aturbojet engine, more particular such a system having an improved airswirler.

Present day turbojet engines are required to generate a high performancelevel, while at the same time maintaining a low pollution rate. Thesecontradictory parameters have required tradeoffs to be made between fullpower operational characteristics to minimize smoke emission andmaximize the life of the engine components, and low speed operationalcharacteristics such as flame stability and engine efficiency. As thepollution standards become increasingly more strict, the combustionchambers' size and the requirement to utilize a diversity of fuelsrenders the aforementioned characteristics increasingly difficult toobtain.

Two module combustion chambers are known wherein one of the chambers isintended for low speed operation and the other for full operation.However, these systems increase the bulk and the weight of the engineand, therefore, are not a complete solution to the problem.

It is also known to use variable geometry injection systems whereindiaphragms or flaps control the air intakes to the combustion chambers.This allows a substantial reduction in the combustion volume and,consequently, the bulk of the chamber.

It is known to utilize this type of variable geometry injection systemswith aerodynamic injectors having intermediate bowl-shaped members. Inthis system, the fuel injectors are mounted in the upstream end of thecombustion chambers with the intermediate bowl-shaped member interposedbetween the fuel injector and the combustion chamber. The bowl-shapedmember typically comprises a shroud which flares outwardly in thedownstream direction and is provided with a plurality of small diameterholes to allow air to enter the atomized fuel cone emanating from theinjector. The bowl shaped member produces turbulent air flow to improvethe fuel atomization in the air-fuel mixture.

In order to further improve the performance characteristics of theseaerodynamic injectors with intermediate bowl-shaped members, the outerswirlers as well as the air intakes of the bowl openings may be equippedwith a diaphragm to control the flow of air through these elements inorder to match the air-fuel mixture richness at the bowl exhaust to alloperating conditions.

U.K. Pat. No. 2,085,147 discloses a typical example of this type ofswirler system in which a control diagrham in the form of a thincylindrical shell surrounds an annular flange defining air intakeorifices. Neither the control diaghragm nor the air intake orifices havevane systems therein to guide and deflect the incoming atomizing air. Inthis particular system, the air guidance is achieved only by means ofcurved extensions on the outside of the control diaghragm.

U.S. Pat. No. 4,534,166 to Kelm et al. show an axial-type swirlerutilized in a combustion chamber structure without having anintermediate bowl shaped member. The slopes of the diaghragm vanesdiffer from those of the swirler structure such that the air-intakeangle can be varied in relation to the operating modes of the engine.However, this system suffers from the drawback that the external swirleris located at the very upstream end of the combustion chamber andsubstantial wakes or turbulence are presented in the fuel cone due tothe location of the swirler vanes. The wakes degrade the air-fuelmixture resulting in less efficient combustion chamber operation. Thesystem shown in Kelm et al. does not permit attenuation of these wakes.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved air-fuelinjection system for a turbojet engine having a combustion chamber, afuel injector and an intermediate bowl-shaped member which defines anair intake duct to direct air into the combustion chamber so as toatomize the fuel emanating from the fuel injector.

A plurality of first vanes are attached to the bowl member and disposedin the air intake duct. A control diaghragm is rotatably mounted to thebowl-shaped member and has a plurality of second vanes which may bemoved into alignment with, or out of alignment with the first vanes. Thefirst vanes are disposed at an angle β with respect to a radius of thebowl shaped member, while the second vanes are disposed at an angle αwith respect to a radius of the bowl-shaped member. According to theinvention, the angles α and β are different from each other in order toenable the swirl angle of the air jet passing through the annual duct tobe varied according to the operational mode of the engine.

The annular air intake duct has a radial inlet portion, in which thefirst and second vanes are disposed, and an axial outlet portion so asto permit the attenuation of the wakes produced by the outer swirlervanes before the air is introduced into the combustion chamber. Theradially innermost or downstream edge of each of the first vanes islocated approximately in radial alignment with the radially outermostportion of the axial outlet portion of the air intake. The first andsecond vanes are oriented such that the magnitude of angle β isapproximately three to four times the magnitude of the angle α. Inaddition, the radial height of the second vanes is approximately threetimes the radial height of the first vanes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial, longitudinal cross sectional view of a turbojetengine combustion chamber equipped with the injection system accordingto the invention.

FIG. 2 is a partial, enlarged cross sectional view of the intermediatebowl-shaped member shown in detail A in FIG. 1.

FIG. 3 is a cross sectional view taken along line I--I in FIG. 2 showingthe control diaghragm in the idle position.

FIG. 4 is a cross sectional view taken along line I--I in FIG. 2 showingthe control diaghragm in its full power position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a partial, longitudinal cross sectional view of an annularcombustion chamber 1 of a turbojet engine. The chamber 1 is locatedbetween an outer casing 2 and an inner shell 3 which serve as boundariesfor the flow of compressed air in known fashion. A fraction F1 of theair emitted from the jet engine compressor (not shown) is guided throughthe injection system 4 to form the atomized air-fuel mixture. Thismixture passes into the primary combustion zone 5 in which thecombustion reaction takes place which produces gases which are, in turn,diluted in zone 6 and cooled in secondary downstream zone 7. The gasesemanating from the combustion chamber pass through a turbine (not shown)before exiting through the exhaust pipe of the engine into theatmosphere.

The fuel injector 8 is indicated in dashed lines in FIG. 1 and isconnected by an intermediate bowl-shaped member 10 to an upstream end 9of the combustion chamber. The injector, in known fashion, may have aninner swirler (not shown) which may be either radial or axial to projectthe fuel from the injector nozzle in the form of a frusto-conical jetflaring outwardly in the downstream direction.

The injector is enclosed by case 11 which also forms the upstream wallof the intermediate bowl-shaped member 10. A downstream portion 11a ofcase 11 defines a frusto-conical portion and is connected to the radialwall portion 11c by a substantially cylindrical portion 11b. Radial wallportion 11c together with radial wall portion 12c of intermediate ring12 defines a radial inlet portion 14a of annular duct 14. Intermediatering 12 also comprises a cylincrical portion 12b which interconnectswith frusto-conical portion 12a, as best seen in FIG. 2. Duct 14 has agenerally axial outlet portion 14a defined by walls 11b, 12b and 11a,12a.

A plurality of first swirler vanes 13 are mounted in the radial inletportion of the annular duct 14 such that their downstream edges 13a arelocated on a circle, the diameter of which is approximately equal to theof the cylindrical portion 12b of intermediate ring 12. A best seen inFIGS. 3 and 4, each of the vanes 13 define an angle β with respect to aradius r₁ of the bowl shaped member 10.

A control diaghragm 15 has a plurality of second vanes and is rotatablymounted on the bowl-shaped member 10 such that the second vanes extendinto the radial inlet portion of annular duct 14. Control diaghragm 15comprises a circular rim having a control lever 16 to which is attachedan actuator (not shown) to rotate the control diaphragm 15 with respectto the bowl-shaped member 10. Each of the second vanes has a radialheight h₂ of approximately three times the radial height h₁ of the firstswirler vanes 13. The rotational position of control diaphragm 15 in thefull power position is shown in FIG. 4. The first swirler vanes 13 arealigned with the second vanes of diaghragm 15. The air passages betweenfirst vanes 13 are substantially completely open to allow the air topass between the plurality of first vanes and the plurality of secondvanes through opening 17 in the direction of arrow 24 shown in FIG. 4.

In response to the operational mode of the turbojet engine, the controldiaghragm can be moved from the full power position to the idle positionshown in FIG. 3. As can be seen, the second plurality of vanes are outof circumferential alignment with the first vanes 13 such that airpassing into the bowl-shaped member is directed in the direction ofarrow 25 shown in FIG. 3 after passing through the openings 17 betweenthe second vanes of the control diaghragm 15.

Each of the vanes of control diaghragm 15 are oriented at an angle αwith respect to a radius r₂ of the bowl-shaped member. Preferably, themagnitude of angle β is between the three and four times the magnitudeof angle α. The angle α preferably has a magnitude of between 20√ and25°, whereas angle β preferably has a magnitude of between 80° and 85°.

At the full power setting shown in FIG. 4, the air passing throughannular duct 14 forms a conical fuel sheet having a relatively narrowapex angle such that the fuel sheet spreads out over a great lengthinside the combustion chamber to optimize the combustion within thechamber. At the idle setting, shown in FIG. 3, the air passing into thecombustion chamber via the annular duct 14 has a greater tangentialangle and therefore undergoes strong centrifuging such that the atomizedfuel cone is modified to include a greater apex angle. As a result, thefuel cone is located very closes to the upstream end of the chamber toimprove the combustion stability under idle conditions.

Tests performed with the injection system according to the inventionhave shown that setting of angle β between 80° and 95° will result inoptimal stability performance at idle, whereas the full power setting isoptimized with angle α equal to between 20° and 25° relative to theradius of the bowl-shaped member.

The bowl-shaped member 10 may also define an impact cooling chamber 22between intermediate ring 12 and bowl nut 20. Impact cooling chamber 22is supplied with air from the engine compressor (not shown) through aplurality of orifices 18 in known fashion.

Diaghragm 15 also may comprise an annular portion 15a which extends overthe air intake orifices 18 and which defines a plurality of openingstherethrough. Thus, as the diaghragm 15 is moved between the idle andfull power positions, the air passing into the impact cooling chamber 22through the intake orifices 18 is also controlled by movement of annularportion 15a.

Orifices 19 are also defined in the downstream portion of intermediatebowl member 10 and allow air to evacuate from the impact cooling chamber22 into the combustion chamber to contribute to the atomization to thefuel. The number of orifices 18 and 19 may be equal to each other or maybe different if their diameters are computed to equalize the output flowwith the input flow when the diaghragm 15a is fully opened.

The fuel injection system according to the invention provides better airguidance and an improved rotational component to the air issuing fromthe external swirlers than the known art discussed above. The airacceleration at the exit from the swirler, due to the intrinsicstructure of the axial duct 14, takes place before the air is introducedat the neck of the bowl-shaped member and serves to attenuate the wakescaused by the external swirler vanes. Therefore, it is possible toachieve a more homogenous combustion volume in the combustion chamberand to avoid overheated zones than in the chamber according to the knownart. Thus, the invention allows the improvement of the service life ofthe combustion chambers while at the same time greatly improving fullpower and idle performance.

The foregoing description is provided for illustrative purposes only andshould not be construed as in any way limiting this invention, the scopeof which is defined solely by the appended claims.

What is claimed is:
 1. In an air-fuel injection system for a turbojetengine having a combustion chamber and at least one fuel injector, theimprovements comprising:(a) a bowl-shaped member interposed between thefuel injector and the combustion chamber, the bowl-shaped memberdefining: an annular air intake duct having a radial inlet portion and agenerally axial outlet portion; an impact cooling chamber; and aplurality of openings to permit fluid communication between the impactcooling chamber and the combustion chamber; (b) A plurality of firstswirler vanes attached to the bowl member and disposed in the radialinlet portion of the annular duct to define a plurality of air passagestherebetween such that the radially innermost edges of the first vanesare approximately radially aligned with a radially outermost part of theaxial outlet portion of the annular duct, each first swirler vaneoriented at an angle β with respect to a radius of the bowl-shapedmember and having a radial height h₁ ; (c) an air control diaphragmrotatably mounted on the bowl shaped member and having a plurality ofsecond swirler vanes, each second swirler vane oriented at an angle α ofbetween 20° and 25° with respect to a radius of the bowl shaped membersuch that the magnitude of β is between three and four times themagnitude of α and having a radial height h₂ and that h₂ isapproximately equal to 3h₁ and , (d) means to move the air controldiaphragm with respect to the bowl shaped member between a firstposition wherein the second swirler vanes are substantiallycircumferentially aligned with the first swirler vanes and a secondposition wherein the second swirler vanes are circumferentiallydisplaced from the first swirler vanes so as to control the direction ofthe air flowing through the air passages between the first vanes intothe annular air intake duct.
 2. The air-fuel injection system accordingto claim 1 wherein the magnitude of angle β is between 80° and 85°. 3.The air-fuel injection system according to claim 1 wherein the number offirst swirler vanes is equal to the number of second swirler vanes. 4.The air-fuel injection system according to claim 1 wherein thebowl-shaped member further defines a plurality of air intake orificesallowing communication between a pressurized air source and the impactcooling chamber.
 5. The air-fuel injection system according to claim 4wherein the air control diaphragm further comprises an annular portionextending over the air intake orifices and defining a plurality ofopenings such that, as the diaphragm is moved relative to thebowl-shaped member, the openings are moved between a first positioncircumferentially aligned with the air intake orifices and secondposition out of circumferential alignment with the air intake orificesso as to control the air flow through the intake orifices.
 6. Theair-fuel injection system according to claim 3 wherein the magnitude ofangle β is between 80° and 85°.
 7. The air-fuel injection systemaccording to claim 6 wherein the number of first swirler vanes is equalto the number of second swirler vanes.